A portmanteauof "retarded" and "petard," it describes a plan backfiring on you in such a way that you are left looking utterly ridiculous. It combines the concept of retarded--which is to say mentally deficient--and the phrase "hoisted by your own petard," which means to have damage done to you by the means you had intended to employ to damage others. A petard is an explosive used to break down barricades in warfare, and so petarded can also mean "blown up," in some circumstances.

You ratted Dan out for stealing that camera, but in doing so proved that you were his accomplice? Dude, you're petarded.

Jim just petarded the front door trying to take out his ex, but the door blew up and a shard of it went through his windshield. Totally petarded.